What should they put to their hearts?
Rashi: To engage in building and learning needs of Kehunah.
Radak: What will be after it is built.
Malbim: (a) They should contemplate how Hashem will conduct with them from now (that they began to build the Mikdash) and onwards - it will change for Brachah and the good. (b) They should put their hearts in Heichal Hashem - the building should not be merely a bodily action, rather, an act of the heart and Nefesh.
What is "va'Ma'alah"?
Rashi: The years that passed.
Radak: From the day that they began building, and onwards.
When is "mi'Terem Sum Even El Even"?
Rashi: Before you returned to add to the foundation that you made in the days of Koresh 1 , and [the building] ceased.
Radak: Think about your deeds (what will be after it is built) before doing them (building). Even El Even is like Even Al Even, just like "El he'Harim Lo Achal" (Yechezkel 18:6) - it is like Al he'Harim.
Radak citing Targum Yonasan: Before they put a row of the building on a row.
Radak: They made the foundation when they built the Mizbe'ach.
